Abstract: A diode circuit in which the flicker noise generated by the diode is reduced comprises a diode (D1) connected in series with a resistor (R2) having a by-pass capacitor (C1) in parallel therewith. In one embodiment the side of the diode remote from this resistor (R2) is connected to one input of a DC differential inverting amplifier (A1) via a low pass filter (L1, C2). The amplifier's output is negatively fed back to the junction of the diode (D1) and the resistor (R2) so that the latter forms the amplifier load. A control voltage is applied to the other input of the amplifier of such value as to produce the required diode bias. The basic diode circuit can be extended to perform functions such as amplitude limiting, frequency control and modulation.

Abstract: A method and apparatus of deriving a directional polar diagram from the signals R received by a synthetic-aperture aerial (in direction-finding applications), or P from such an aerial (in radio-beacon applications), these signals including a carrier of frequency .omega. which is phase-modulated at the scanning frequency .omega..sub.m of the individual aerials. The method comprises generating an "estimated" signal (26) of frequency .omega. phase-modulated at frequency .omega..sub.m, and multiplying (25) the generated signal by the received signal to derive an output which includes a selectable Bessel Function (J.sub.O) representing the directional polar diagram. The phase (with circular aerial arrays) (27) or amplitude (with linear aerial arrays) of .omega..sub.m as applied to phase-modulate the generated signal is controlled to maximize or minimize, as appropriate, the selected Bessel Function and thereby control the bearing direction of the polar diagram. The zero-order Function J.sub.

Abstract: An output having reduced additives phase-noise in at least one defined frequency band is obained from a voltage-controlled oscillator (5) by applying a frequency-modulating signal .omega..sub.m t to the oscillator to produce a sideband within the defined band, the signal having a frequency much higher than the highest multiplicative phase-noise frequency (referred to the unmodulated oscillator frequency) in that band and producing a modulaton index .DELTA.P much greater than the modulation index .DELTA.P.sub.n of the total phase-noise (i.e. additive plus multiplicative)in that band. The oscillator output is then phase-modulated with a signal at .omega..sub.